Method and apparatus for flight control of tiltrotor aircraft

ABSTRACT

A method and apparatus provide for automatically controlling the flight of a tiltrotor aircraft while the aircraft is in flight that is at least partially rotor-borne. The method and apparatus provide for automatically tilting nacelles in response to a longitudinal-velocity control signal so as to produce a longitudinal thrust-vector component for controlling longitudinal velocity of the aircraft. Simultaneously, cyclic swashplate controls are automatically actuated so as to maintain the fuselage in a desired pitch attitude. The method and apparatus also provide for automatically actuating the cyclic swashplate controls for each rotor in response to a lateral-velocity control signal so as to produce a lateral thrust-vector component for controlling lateral velocity of the aircraft. Simultaneously, collective swashplate controls for each rotor are automatically actuated so as to maintain the fuselage in a desired roll attitude. The method and apparatus provide for yaw control through differential longitudinal thrust produced by tilting the nacelles.

TECHNICAL FIELD

The present invention relates in general to the field of flight controlof aircraft. In particular, the present invention relates to apparatusand methods for controlling the flight of a tiltrotor aircraft.

DESCRIPTION OF THE PRIOR ART

A rotary wing aircraft, such as a helicopter or the tiltrotor aircraft11 shown in FIG. 1, produces lift with at least one main rotor 13, whichcomprises multiple wings, or blades 15, attached to a rotating hub 17.Each blade 15 has an airfoil cross-section, and lift is produced bymoving blades 15 in a circular path as hub 17 rotates. As shown in thefigures, the left and right sides of aircraft 11 are generally mirrorimages of each other, having corresponding components on each side ofaircraft 11. As described herein, a single reference number may be usedto refer to both left and right (as viewed if seated in the aircraft)components when the description applies to both components. Specificreference numbers are used for clarity to refer to specific left orright components when the description is specific to either the left orright component. For example, “rotor 13” may be used in descriptions ofboth the left rotor and the right rotor, and “rotor 13A” and “rotor 13B”may be used in descriptions that are specific to the left and rightrotors, respectively.

The amount of lift produced can be varied by changing the angle ofattack, or pitch, of blades 15 or the speed of blades 15, though thespeed of rotor 13 is usually controlled by use of a RPM governor towithin a narrow range for optimizing performance. Varying the pitch foreach blade 15 requires a complex mechanical system, which is typicallyaccomplished using a swashplate assembly (not shown) located on each hub17.

Each swashplate assembly has two primary roles: (1) under the directionof the collective control, each swashplate assembly changes the pitch ofblades 15 on the corresponding rotor 13 simultaneously, which increasesor decreases the lift that each rotor 13 supplies to aircraft 11,increasing or decreasing each thrust vector 19 for causing aircraft 11to gain or lose altitude; and (2) under the direction of the cycliccontrol, each swashplate assembly changes the angle of blades 15 on thecorresponding rotor 13 individually as they move with hub 17, creating amoment in a generally horizontal direction, as indicated by arrows 21,for causing aircraft 11 to move in any direction around a horizontal360-degree circle, including forward, backward, left and right.

Typically, the collective blade pitch is controlled by a lever that thepilot can move up or down, whereas the cyclic blade pitch is controlledby a control stick that the pilot moves in the direction of desiredmovement of the aircraft. The collective control raises the entireswashplate assembly as a unit, changing the pitch of blades 15 by thesame amount throughout the rotation of hub 17. The cyclic control tiltsthe swashplate assembly, causing the angle of attack of blades 15 tovary as hub 17 rotates. This has the effect of changing the pitch ofblades 15 unevenly depending on where they are in the rotation, causingblades 15 to have a greater angle of attack, and therefore more lift, onone side of the rotation, and a lesser angle of attack, and thereforeless lift, on the opposite side of the rotation. The unbalanced liftcreates a moment that causes the pitch or roll attitude of aircraft 11to change, which rotates the thrust vectors and causes aircraft 11 tomove longitudinally or laterally.

A tiltrotor aircraft, such as aircraft 11, also has movable nacelles 23that are mounted to the outer ends of each fixed wing 25. Nacelles 23can be selectively rotated, as indicated by arrows 27, to any pointbetween a generally vertical orientation, as is shown in FIG. 1,corresponding to a “helicopter mode” for rotor-borne flight using blades15 to provide lift, and a horizontal orientation, corresponding to an“airplane mode” for forward flight using fixed wings 25 to produce lift.Aircraft 11 may also operate in partial helicopter mode at low speeds,in which rotors 13 and fixed wings 25 both provide part of the requiredlift for flight. The operation of aircraft 11 typically includes avertical or short takeoff, a transition from helicopter mode to airplanemode for forward flight, and then a transition back to helicopter modefor a vertical or short landing.

Due to the many variables involved in the control of flight of atiltrotor aircraft, a computer-controlled flight control system (FCS) 28automates many of the functions required for safe, efficient operation.FCS 28 actuates flight-control components of aircraft 11 in response tocontrol inputs generated by one or more of the following: (1) anon-board pilot; (2) a pilot located remote from the aircraft, as with anunmanned aerial vehicle (UAV); (3) a partially autonomous system, suchas an auto-pilot; and (4) a fully autonomous system, such as in an UAVoperating in a fully autonomous manner. FCS 28 is provided withsoftware-implemented flight control methods for generating responses tothese control inputs that are appropriate to a particular flight regime.

In the automatic control methods of current tiltrotor aircraft, when acommand for a change in longitudinal velocity is received by FCS 28while aircraft 11 is in full or partial helicopter mode, FCS 28 induceslongitudinal acceleration of aircraft 11 by changing the pitch attitudeof aircraft 11 to direct thrust vectors 19 forward or rearward. Thechange of pitch attitude is accomplished by FCS 28 commanding theswashplates to tilt forward or rearward using cyclic control, whichcauses aircraft 11 to pitch downward in the direction that the aircraftis commanded to fly. For example, when aircraft 11 is commanded by apilot to fly in the forward direction by moving the cyclic controlforward, FCS 28 commands the swashplate for each rotor 13 to tiltforward, and rotors 13 create a forward pitch moment. As shown in FIG.2, the moment causes the plane of blades 15 to tilt forward and alsopitches aircraft 11 in the nose-down direction, which is visible incomparison to ground 29. Thrust vectors 19 are thus rotated toward theforward direction, and the result is movement in the direction shown byarrow 30.

There are several undesirable influences on aircraft 11 using thisflight control method, especially in a gusty or windy environment. Whenthe pitch attitude of aircraft 11 is changed due to a command to move inthe forward/rearward direction, there is a change in the angle of attackof wings 25 and a corresponding reduction in lift produced by wings 25,and this may produce an undesirable change in the vertical velocityand/or altitude of aircraft 11, which must be countered by changing thevertical climb command. This pitch-attitude-to-vertical-velocitycoupling is especially true when hovering or in a low-speed flightcondition, and is more pronounced in the presence of a headwind. Usingthe current automatic flight control method in this situation, aircraft11 cannot accelerate in the forward direction without a nose-down pitchattitude, and the resulting uncommanded and unwanted vertical motioninterferes with the precise vertical control of aircraft 11.

In the automatic control methods of current tiltrotor aircraft, when acommand for a change in lateral velocity is received by FCS 28 while theaircraft is in full or partial helicopter mode, FCS 28 induces lateralacceleration of aircraft 11 by changing the roll attitude of aircraft 11to direct thrust vectors 19 to the left or right. This is accomplishedusing differential collective blade pitch control, which causes fuselage23 to tilt right or left in the direction that aircraft 11 is commandedto fly. For example, when aircraft 11 is commanded to fly to the right,FCS 28 commands the collective controls on rotors 13 such that rightrotor 13 produces less lift than that being produced by left rotor 13.The resulting thrust imbalance causes aircraft 11 to roll to the right,as shown in FIG. 3, directing thrust vectors 19 to the right and causingaircraft 11 to move in the direction of arrow 31.

This automatic flight control method of tilting aircraft 11 duringlateral maneuvering also causes several problems. When aircraft 11 isoperating in the area of ground effects, which it must do each time itis in close proximity to a large surface, such as ground 29 duringtakeoff and landing, the rolling of aircraft 11 will cause one rotor 13to be closer to ground 29 than the other rotor 13. This difference inrelation to ground 29 will cause the ground effects to be greater on oneside of aircraft 11 than on the other, which will cause the lift of eachrotor 13 to change differently. This difference will cause an additionalroll moment on aircraft 11, and this interferes with the precise controlof aircraft 11. The rolling of aircraft 11 also tends to blow the aircushion out from under one side of aircraft 11, further degrading thecontrollability.

When aircraft 11 is moving laterally, or is hovering in a sideward wind,and wings 25 are tilted to the left or right, there is more drag or windresistance. There is also an increase in down loading, which is theloading of the top of wings 25 by the dynamic pressure caused by rotors13 and the lateral aircraft velocity. Both of these conditions degradethe controllability in the lateral and vertical axes and require morepower than flying level in the same wind conditions.

Aircraft 11 is also subject to upsets from wind gusts, with wind fromany direction causing large position displacements when using thecurrent control methods. For example, if aircraft 11 experiences a windgust from the left side, aircraft 11 will roll to the right. Whenaircraft 11 rolls to the right, thrust vectors 19 are also rotated tothe right, which makes the lateral velocity of aircraft 11 increase tothe right. In current tiltrotor aircraft, if FCS 28 is programmed tohold the aircraft over a specified point on the ground, FCS 28 willcommand aircraft 11 to roll back to the left, causing thrust vectors 19to oppose the gust and to move aircraft 11 back to the position itoccupied before the gust. This method of control has the disadvantage ofallowing the gust to displace aircraft 11 a significant distance fromits original position before FCS 28 can drive aircraft 11 back to theoriginal position.

Other problems with the current methods of control include high responsetime to FCS commands and reduced passenger comfort. Response time toforward and lateral velocity commands is high due to the requirementthat the attitude of aircraft 11 change for these commands to beexecuted, and the high inertia of a large, manned tiltrotor, such asaircraft 11, translates into low response frequencies of the system. Asignificant disadvantage for tiltrotors used to carry passengers is thatpassenger comfort is compromised by tilting fuselage 23 of aircraft 11while maneuvering while hovering or in low-speed flight, such as whileapproaching for a landing and when moving aircraft 11 into position toaccelerate to forward flight.

In the automatic control methods of current tiltrotor aircraft, when acommand to change the yaw velocity (i.e., the velocity of change ofheading) of aircraft 11 is received by FCS 28 while the aircraft is infull or partial helicopter mode, FCS 28 induces a yawing moment usingdifferential longitudinal cyclic control. For example, when aircraft 11is commanded to yaw to the left, such as when a pilot depresses the leftrudder pedal, FCS 28 commands the swashplate for right rotor 13B to tiltforward and commands the swashplate of left rotor 13A to tilt rearward.As shown in FIG. 4, the planes of blades 15A and 15B and the directionof thrust vectors 19A, 19B are tilted in opposite directions, withvector 19A having a rearward thrust component and vector 19B havingforward thrust component. Thrust vectors 19A, 19B create a yaw moment,resulting in rotation of aircraft 11 generally about a vertical yaw axis32 in the direction shown by arrow 33.

SUMMARY OF THE INVENTION

There is a need for an improved apparatus and improved methods forcontrolling tiltrotor aircraft with minimized tilting of the fuselage ofthe aircraft and enhanced accuracy of control.

Therefore, it is an object of the present invention to provide animproved apparatus and improved methods for controlling tiltrotoraircraft.

The present invention provides a flight control system (FCS)implementing the control methods of the invention for automatic flightcontrol of a tiltrotor aircraft while operating at low airspeeds or in ahover, especially during operation in gusty and turbulent windconditions. In response to a control input for a change in longitudinalvelocity, such as a pilot pushing forward on the cyclic control, the FCScommands the nacelles to rotate in the same direction for directingthrust vectors of the rotors in a longitudinal direction.Simultaneously, the FCS automatically holds the fuselage at a desiredpitch attitude by use of the longitudinal cyclic swashplate controls.

In response to a control input for a change in lateral velocity, such asa pilot pushing sideways on the cyclic control, the FCS commands thelateral cyclic swashplate controls for directing thrust vectors of therotors in a lateral direction. Simultaneously, the FCS automaticallyholds the fuselage to a desired roll attitude by differential use ofrotor collective controls.

In response to a control input for a change of yaw velocity, such as apilot depressing a rudder pedal, the FCS commands the nacelles to rotatefor directing thrust vectors of the rotors in different directions,creating a moment that causes the aircraft to yaw.

The present invention provides significant advantages over the priorart, including: (1) providing longitudinal and lateral velocity controlwhile maintaining the fuselage in a desired attitude; (2) reducingresponse time to forward and lateral velocity commands; (3) increasingaccuracy of aircraft control; (4) reducing position displacements causedby wind gusts; (5) reducing the pitch-attitude to vertical-velocitycoupling; (6) reducing the responses to ground effects; and (7) reducingthe power required for lateral flight.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, includingits features and advantages, reference is now made to the detaileddescription of the invention taken in conjunction with the accompanyingdrawings in which like numerals identify like parts, and in which:

FIG. 1 is a perspective view of a prior-art tiltrotor aircraft;

FIG. 2 is a side view of the tiltrotor aircraft of FIG. 1 executing acommand to fly forward using a prior-art control method;

FIG. 3 is a front view of the tiltrotor aircraft of FIG. 1 executing acommand to fly to the right using a prior-art control method;

FIG. 4 is a side view of the tiltrotor aircraft of FIG. 1 executing acommand to yaw to the left using a prior-art control method;

FIG. 5 is a side view of a tiltrotor aircraft using apparatus andcontrol methods according to the present invention to maintain positionin a hover;

FIG. 6 is a side view of the tiltrotor aircraft of FIG. 4 executing acommand to fly forward using a control method according to the presentinvention;

FIG. 7 is a front view of the tiltrotor aircraft of FIG. 4 using acontrol method according to the present invention to maintain positionin a hover;

FIG. 8 is a front view of the tiltrotor aircraft of FIG. 4 executing acommand to fly to the right using a control method according to thepresent invention;

FIG. 9 is a side view of the tiltrotor aircraft of FIG. 4 executing acommand to yaw to the left using a control method according to thepresent invention;

FIG. 10 is a perspective view of an unmanned tiltrotor aircraftaccording to the present invention; and

FIG. 11 is a perspective view of a civilian passenger version of atiltrotor aircraft according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 5, a tiltrotor aircraft 34 is depicted in a hoverabove ground 35. Aircraft 34 is constructed in the same manner asaircraft 11, described above, but the flight control system (FCS) 36 inaircraft 34 uses the control methods of the present invention toautomatically control the flight of aircraft 34 in response to controlinputs by a pilot or electronic system. Rotors 37, comprising hub 39 andmultiple blades 41, are powered by engines carried within nacelles 43.Nacelles 43 are rotatably mounted to the outer ends of wings 45, andwings 45 are affixed to fuselage 47. As described above, the pitch ofeach blade 41 is controlled by collective and cyclic swashplate controls(not shown) located within hub 39. As described herein, a singlereference number may be used to refer to both left and right components(as viewed when seated in the aircraft) when the description applies toboth components. Specific reference numbers are used for clarity torefer to specific left or right components when the description isspecific to either the left or right component.

In the method of the present invention, a control input for a change inlongitudinal velocity, such as a pilot pushing forward or pullingrearward on the cyclic control, causes FCS 36 to command nacelles 43 torotate in the same direction for directing thrust vectors 49 of rotors37 in a longitudinal direction. Simultaneously, FCS 36 automaticallyholds the pitch attitude of fuselage 47 to a desired pitch attitude,which may be a generally level pitch attitude, by use of thelongitudinal cyclic swashplate controls. For example, FIG. 6 showsaircraft 34 configured for forward motion, with nacelles 43 tiltedforward to give each thrust vector 49 a forward vector component. Thesecomponents tends to drive aircraft 34 forward in the direction shown byarrow 51, while the swashplate controls in each rotor 37 are used tocontrol the pitch attitude of fuselage 47. In addition to a response toa control input, FCS 36 can generate commands in response to alongitudinal position error, in which nacelles 43 are commanded so as toreturn aircraft 34 to a previous position or to fly to a selectedposition.

This longitudinal velocity control method differs from the prior-artcontrol method in that change of the pitch attitude of fuselage 47 isnot required to change the longitudinal velocity of aircraft 34.Maintaining a generally level pitch attitude prevents the angle ofattack for wings 45 from changing and prevents the undesirable change invertical forces that cause problems in controlling the vertical aircraftposition using the prior-art control methods. Specifically, whenhovering or in a low-speed flight condition, especially in the presenceof a headwind, the longitudinal velocity control method of the presentinvention will reduce the pitch-attitude to vertical-velocity couplingby allowing aircraft 34 to accelerate in the forward direction without anose-down pitch attitude. In addition, the method of the presentinvention allows the attitude of aircraft 34 to be controlled to themost favorable condition during the conversion from helicopter mode toairplane mode.

The control methods of the present invention also include an improvedmethod of lateral velocity control of aircraft 34, the method beingimplemented in FCS 36. Aircraft 34 is shown in a hover above ground 35in FIG. 7, with the left rotor labeled as 37A and the right rotorlabeled as 37B. Each rotor 37A, 37B produces a vertical thrust vector49A, 49B, respectively, for lifting aircraft 34. In response to acontrol input for a change in lateral velocity, such as a pilot pushingsideways on the cyclic control, FCS 36 commands the lateral cyclicswashplate controls for directing thrust vectors 49A, 49B of rotors 37A,37B in a lateral direction. Simultaneously, FCS 36 automatically holdsthe roll attitude of fuselage 47 in a desired roll attitude, which maybe a generally level roll attitude, by differential use of rotorcollective controls. In addition to a response to a control input, FCS36 can generate commands in response to a lateral position error, inwhich the lateral cyclic swashplate controls are commanded so as toreturn aircraft 34 to a previous position or to fly to a selectedposition.

For example, FIG. 8 shows aircraft configured for movement to the right(as viewed if seated in the aircraft). When command to move to theright, swashplate controls tilt the plane of rotors 37A, 37B to theright, causing thrust vectors 49A, 49B to have a horizontal component tothe right, and this vector component causes aircraft 34 to move in thedirection shown by arrow 53. While the cyclic swashplate controls inducesideward movement, the differential collective blade control is used tohold the aircraft level, meaning that the collective controls for rotors37A, 37B are actuated independently from each other to maintain thedesired fuselage attitude. This combination of controls allows aircraft34 to move laterally in a stable and precise manner while holdingaircraft 34 in a level roll attitude. A key advantage to the controlmethod of the present invention is that holding fuselage 47 in a levelattitude during lateral flight minimizes ground-effect problems and wingdown-loading problems encountered when rolling aircraft 34 using theprior-art method.

Additionally, the lateral velocity control method of the inventionprovides for improved lateral gust response, which may be reduced by asmuch as around 80%. When a lateral gust hits aircraft 34, FCS 36 willimmediately command the lateral cyclic swashplate control in thedirection opposing the gust while the differential collective bladecontrol is commanded to hold aircraft 34 level. Aircraft 34 will stillhave a tendency to roll with the gust, but thrust vectors 49A, 49B canquickly be redirected to oppose the gust without the need to rollaircraft 34 beyond the amount required to bring aircraft 34 back to agenerally level roll attitude or other desired roll attitude. Asdescribed above, FCS 36 may also generate commands to the cyclicswashplate controls in response to a lateral position error forreturning aircraft 34 to the position aircraft 34 occupied prior to thedisplacement caused by the gust.

The swashplate cyclic controls are limited by physical constraints andthe geometry of the system, such that there is a limited amount of totalcyclic allowed for all cyclic command inputs. The total cyclic used atany one time is the square root of the sum of the squares of thelongitudinal cyclic and the lateral cyclic. As described above, themethods of the invention include using longitudinal cyclic controls forcontrolling the aircraft pitch attitude and using lateral cycliccontrols for controlling the lateral velocity of the aircraft.Longitudinal cyclic is also required to control the aircraft pitchmoment as the location of the center of gravity of aircraft 34 changes.To reduce the total cyclic swashplate commands, the present inventionalso includes a control method for controlling yaw in aircraft 34without the requirement of using longitudinal cyclic controls.

The yaw control method provides for differential nacelle control, inwhich nacelles 43 of aircraft 34 are rotated independently to directtheir thrust vectors 49 in different directions, creating a yaw moment.For example, FIG. 9 shows aircraft 34 configured for yawing in adirection with the nose of aircraft 34 moving to the left (as viewed ifseated in the aircraft). Left nacelle 43A has been rotated rearward, andright nacelle 43B has been rotated forward, directing thrust vectors 49Aand 49B in different directions. Thrust vector 49A has a longitudinalthrust component pointing toward the rear of aircraft 34, and thrustvector 49B has a longitudinal thrust component pointing toward the frontof aircraft 34. This longitudinal thrust differential creates a yawmoment, causing aircraft 34 to rotate in the direction of arrow 55 abouta yaw axis 57. An advantage of this yaw control method is that removingthe yaw control commands from the total cyclic commands provides formore cyclic control range to be available for control of pitch attitude,center-of-gravity changes, and lateral aircraft velocity control. Thisallows for increased longitudinal center-of=gravity range, increasedcapability to hover in a crosswind, increased maneuver envelope for thepitch, roll, and yaw axes, reduced rotor flapping, and simplifiedprioritization of cyclic commands. Also, the yaw control is not limitedby cyclic authority limits.

While shown in FIGS. 5-9 as used with a manned, military-style aircraft34, the improved FCS and control methods of the present invention mayalso be applied to control any type of tiltrotor aircraft. FIG. 10 showsan unmanned aerial vehicle 59 (UAV) constructed as a tiltrotor aircraft.The enhanced accuracy of control permitted by the methods of the presentinvention is especially beneficial with the remote and often automatedoperation of UAVs. Specific functions that are enabled or enhancedinclude automatic launch and automatic recovery from a secondaryvehicle, such as from the deck of a ship at sea, and maneuvering arounda particular location or target in windy conditions with the requiredaccuracy. Also, the reduced response time to forward and lateralvelocity commands provides for a greater maneuver bandwidth, which is agreat advantage for automatically controlled aircraft.

A civilian passenger version of a tiltrotor aircraft 61 is depicted inFIG. 11. As discussed above, the advantages realized from using thecontrol methods of the invention include improved passenger comfort. Byholding aircraft 61 in generally level pitch and roll attitudes whilemaneuvering in hover or low-speed flight, the passengers aboard aircraft57 are not subjected to the tilting and associated change of relativedirection of acceleration due to gravity, or g-forces, felt when usingthe prior-art methods of control.

The present invention provides significant advantages over the priorart, including: (1) providing longitudinal and lateral velocity controlwhile maintaining the fuselage in a desired attitude; (2) reducingresponse time to forward and lateral velocity commands; (3) increasingaccuracy of aircraft control; (4) reducing position displacements causedby wind gusts; (5) reducing the pitch-attitude to vertical-velocitycoupling; (6) reducing the responses to ground effects; and (7) reducingthe power required for lateral flight.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. For example, it should be appreciated that these controlmethods may also be applicable to other tiltrotor aircraft, such as aQuad tiltrotor aircraft having four nacelles.

The invention claimed is:
 1. A method for automatically controlling theflight of a tiltrotor aircraft while the aircraft is in flight that isat least partially rotor-borne, the tiltrotor aircraft being capable offlying in a helicopter mode and in an airplane mode, the aircraft havinga fuselage and at least two tiltable nacelles, each nacelle having arotor with adjustable-pitch blades controlled by cyclic swashplatecontrols and collective swashplate controls, the method comprising thesteps of: providing a flight control system; providing longitudinalswashplate controls in each nacelle; providing lateral swashplatecontrols in each nacelle; operating the tiltrotor aircraft in thehelicopter flight mode while the fuselage is positioned at a level pitchattitude; generating a longitudinal-velocity control signal based on apilot control input for generating a longitudinal velocity whilemaintaining the aircraft in the helicopter flight mode; automaticallytilting the nacelles with the flight control system in response to thelongitudinal-velocity control signal so as to produce a longitudinalthrust-vector component for controlling longitudinal velocity of theaircraft; automatically actuating the longitudinal cyclic swashplatecontrols for each rotor with the flight control system so as to maintainthe fuselage in the level pitch attitude; and maintaining the fuselagein the level pitch attitude; wherein each nacelle is independentlytiltable; wherein the automatically actuating the longitudinal cyclicswashplate controls for each rotor counteracts a changing in pitchattitude that would otherwise result from tilting the nacelles; andwherein the counteracting the changing in pitch attitude occurssimultaneously with the automatically tilting the nacelles.
 2. Themethod according to claim 1, further comprising the steps of: providingcollective swashplate controls in each nacelle; generating alateral-velocity control signal; automatically actuating the lateralcyclic swashplate controls for each rotor with the flight control systemin response to the lateral-velocity control signal so as to produce alateral thrust-vector component for controlling lateral velocity of theaircraft; and automatically actuating the collective swashplate controlsfor each rotor with the flight control system so as to maintain thefuselage in a level roll attitude; wherein the collective swashplatecontrols for each rotor are actuated independently from each other.
 3. Amethod for automatically controlling the flight of a tiltrotor aircraftwhile the aircraft is in flight that is at least partially rotor-borne,the tiltrotor aircraft being capable of flying in a helicopter mode andin an airplane mode, the aircraft having a fuselage and at least twotiltable nacelles, each nacelle having a rotor with adjustable-pitchblades controlled by cyclic swashplate controls and collectiveswashplate controls, the method comprising the steps of: providing aflight control system; providing longitudinal swashplate controls ineach nacelle; providing lateral swashplate controls in each nacelle;operating the tiltrotor aircraft in the helicopter flight mode while thefuselage is positioned at a level roll attitude; generating alateral-velocity control signal based on a pilot control input forgenerating a lateral velocity while maintaining the aircraft in thehelicopter flight mode; automatically actuating the lateral cyclicswashplate controls for each rotor with the flight control system inresponse to the lateral-velocity control signal so as to produce alateral thrust-vector component for controlling lateral velocity of theaircraft, thereby resulting in a changing in roll attitude of thefuselage from the level roll attitude; and automatically differentiallyactuating the collective swashplate controls for each rotor with theflight control system in response to the lateral-velocity control signalso as to maintain the fuselage in the level roll attitude; wherein thecollective swashplate controls for each rotor are actuated independentlyfrom each other; wherein each nacelle is independently tiltable fromeach other; wherein the automatically differentially actuating thecollective swashplate controls for each rotor prevents the changing inroll attitude of the fuselage; and wherein the preventing the changingin roll attitude occurs simultaneously with the automatically actuatingthe lateral cyclic swashplate.
 4. The method according to claim 3,further comprising the steps of: generating a longitudinal-velocitycontrol signal; automatically tilting the nacelles with the flightcontrol system in response to the longitudinal-velocity control signalso as to produce a longitudinal thrust-vector component for controllinglongitudinal velocity of the aircraft; and automatically actuating thelongitudinal cyclic swashplate controls for each rotor with the flightcontrol system so as to maintain the fuselage in a level pitch attitude.5. A method for controlling a response of a tiltrotor aircraft to a windgust while the aircraft is in flight that is at least partiallyrotor-borne, the tiltrotor aircraft being capable of flying in ahelicopter mode and in an airplane mode, the aircraft having at leasttwo tiltable nacelles, each nacelle having a rotor with adjustable-pitchblades controlled by cyclic swashplate controls and collectiveswashplate controls, the method comprising: providing a flight controlsystem; providing longitudinal swashplate controls in each nacelle;providing lateral swashplate controls in each nacelle; operating thetiltrotor aircraft in the helicopter flight mode while the fuselage ispositioned at a level pitch attitude and a level roll attitude;automatically tilting the nacelles with the flight control system so asto produce a longitudinal thrust-vector component that opposes alongitudinal component of the wind gust while maintaining the aircraftin the helicopter flight mode; automatically actuating the lateralcyclic swashplate controls for each rotor with the flight control systemso as to produce a lateral thrust-vector component that opposes alateral component of the wind gust and to maintain the fuselage in thelevel pitch attitude, thereby preventing a changing in pitch attitude ofthe fuselage that would otherwise result from the tilting the nacellesto produce the longitudinal thrust-vector component; and automaticallyactuating the collective swashplate controls for each rotor with theflight control system so as to maintain the fuselage in the level rollattitude, thereby preventing a changing in roll attitude of the fuselagethat would otherwise result from the actuating of the cyclic swashplatecontrols so as to produce the lateral thrust-vector component; whereineach nacelle is independently tiltable; wherein the wind gust issufficient to positionally displace the tiltrotor aircraft; whereinautomatically tilting the nacelles reduces position displacement by thewind gust; wherein automatically actuating the lateral cyclic swashplatecontrols reduces position displacement by the wind gust; and whereinautomatically actuating the collective swashplate controls reducesposition displacement by the wind gust by differential collectivecontrol.
 6. A method for automatically controlling the flight of atiltrotor aircraft while the aircraft is in flight that is at leastpartially rotor-borne, the tiltrotor aircraft being capable of flying ina helicopter mode and in an airplane mode, the aircraft having only afirst tiltable nacelle and a second tiltable nacelle both nacelleslocated on outer ends of a fixed wing, each nacelle having a rotor, themethod comprising the steps of: providing a flight control system havingcyclic authority limits; generating a yaw-control signal based upon on apilot control input for generating a yaw moment while maintaining theaircraft in the helicopter flight mode; automatically tilting the firsttiltable nacelle and the second tiltable nacelle with the flight controlsystem in response to the yaw-control signal so as to produce alongitudinal thrust differential between the rotors for controlling yawvelocity of the aircraft, the longitudinal thrust differential betweenthe rotors being a result of the first tiltable nacelle being rotatedtoward a front of the aircraft with the second tiltable nacelle beingrotated toward a rear of the aircraft; and wherein each nacelle isindependently tiltable; wherein the first nacelle and the second nacelleare on opposite sides of a longitudinal centerline of the fuselage; andwherein the yaw-control signal is not limited by the cyclic authoritylimits.